This application is based upon and claims the benefit of priority of Japanese Patent Application No. 2000-139702 filed on May 12, 2000, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a fuel injection valve for an internal combustion engine (hereinafter called as an engine).
2. Description of Related Art
As shown in FIG. 6, a conventional fuel injection valve 100 has a nozzle needle 101 that is slidably and reciprocatingly supported by a sliding portion 102. The sliding portion 102 is provided with a plurality of cuts for forming fuel passages. The cuts of the sliding portion 102 also serve as vapor passages through which vapor generated by heat near injection holes moves toward a side of fuel upstream. A contact portion 103 formed at an end of the nozzle needle 101 on a side of fuel injection may be seated on a valve seat 105a formed in a valve body 105.
An armature 111 is connected with the nozzle needle 101 at a position facing a stator 110 and is biased in a valve closing direction by a spring 112. Since the armature 111 and the sliding portion 102 are slidably and reciprocatingly supported by the valve body 105, the nozzle needle 101 can make a reciprocating movement accurately along a center axis thereof. Generally, the stator 110 and the armature 111 are made of lower toughness material and are plated with, for example, chromium to form thin film thereon. When a coil 115 is energized, the armature 111 is attracted toward the stator 110 against biasing force of the spring 112. Accordingly, the nozzle needle 101 leaves the valve seat 105a so that fuel is injected from the injection holes. When the coil 115 is de-energized, the contacting portion 103 is seated on the valve seat 105a to finish fuel injection.
It is important for better fuel consumption that fuel is supplied to the engine at an adequate timing during a period when an intake port of the engine is opened. Therefore, the fuel injection valve is required to have quick response characteristic that is largely affected by mass of a moving member including the nozzle needle 101.
According to the conventional fuel injection valve 100, the nozzle needle 101 is integrally provided with the sliding portion 102 having the cuts, whose maximum outer diameter is larger than that of the contact portion 103, for securing the fuel and vapor passages. Accordingly, among the nozzle needle 101, the sliding portion 102 and the armature 111, which constitute the moving member, the sliding portion 102 adversely affects on the quick response characteristic of the fuel injection valve because of larger mass thereof.
Further, formation of the chromium thin film on portions where the stator 110 and the armature 111 come in contact with each other results in higher manufacturing cost of the fuel injection valve.
An object of the invention is to provide a fuel injection valve in which weight of a nozzle needle is relatively light and mass of a movable member constituted by the nozzle needle and an armature is smaller so that quicker response characteristic of the injection valve is secured.
To achieve the above object, in the injection valve having a housing, a stator, an armature and a coil for exerting electromagnetic attracting force on the armature, a cylindrical valve body, which is provided with at least an injection hole, has a valve seat protruding radially inward out of an inner wall thereof, which is positioned on a side of the stator with respect to the injection hole, and a needle supporting cylindrical inner wall, which is positioned on a side of the stator with respect to the valve seat. A nozzle needle is fixed to the armature so as to move together with the armature in the cylindrical valve body, while being supported slidably by the needle supporting cylindrical inner wall. The nozzle needle is provided with a valve portion to be seated on the valve seat when the coil is de-energized and inside thereof with a cavity into which fuel is introduced. A fuel accumulation bore is provided between inner circumference of the cylindrical valve body extending axially from the valve seat to the needle supporting cylindrical inner wall and outer circumference of the nozzle needle.
With the construction mentioned above, the nozzle needle is further provided with an opening through which the cavity communicates with the fuel accumulation bore so that, when the valve portion leaves the valve seat upon energizing coil, the fuel accumulation bore communicates with the injection hole for fuel injection.
Since the nozzle needle is provided inside thereof with the cavity and with the opening through which the cavity communicates with the fuel accumulation bore, weight of the nozzle needle is lighter than that of the conventional fuel injection valve in which the nozzle needle has the cuts for forming the fuel and vapor passages between the armature accommodation bore and the fuel accumulation bore.
Preferably, the opening of the nozzle needle is opened to the highest position in the fuel accumulation bore to evacuate vapor smoothly.
It is preferable that the housing has a hollow into which fuel is flown from outside and the stator is provided with a penetrating bore communicating with the hollow of the housing at an axial end thereof and communicating with an armature accommodation bore at another axial end thereof, and the armature has a through-hole for making the armature accommodation bore on a side of the stator communicate with the cavity so that fuel is introduced from the hollow of the housing into the cavity. With this construction, the fuel injection valve becomes further lighter and more compact.
It is preferable that the nozzle needle penetrates axially along the through-hole of the armature until an axial end thereof protrudes out of an axial end of the armature toward the stator so that fuel is introduced into the cavity from the hollow of the housing via the penetrating bore. This will make it possible to manufacture the fuel injection valve at lower cost, since an air gap is automatically formed between the stator and the armature by the axial end of the nozzle needle protruding out of the end of the armature and coming in contact with the stator and, further, it is not necessary to cover the axial end of the nozzle needle with chromium thin film for reinforcement because the nozzle needle is inherently made of material having relatively higher stiffness.
Preferably, the opening of the nozzle needle is formed to axially stride over the needle supporting cylindrical inner wall so that the cavity communicates not only with the fuel accumulation bore but also with the armature accommodation bore on a side of the cylindrical valve body. With this construction, vapor generated by heat is easily evacuated from the fuel accumulation bore to the armature accommodation bore through the opening. Accordingly, fluctuation of injection characteristic due to vapor is limited.
Further, it is preferable that the needle cylindrical inner wall, whose diameter is larger than a diameter of the valve seat, is formed to protrude radially inward out of the inner wall of the cylindrical valve body. Since a diameter of the valve seat are smaller than that of the needle supporting cylindrical inner wall, the seat valve, on which the valve portion of the nozzle needle is seated, is easily and accurately machined by inserting a cutting tool from a side of the needle supporting cylindrical inner wall into an inside of the cylindrical valve body.
Moreover, preferably, the nozzle needle is provided with a small diameter column portion whose axial end on a side of the injection hole constitutes the valve portion and with a large diameter column portion whose diameter is larger than that of the small diameter column portion and which is slidably supported by the needle supporting cylindrical inner wall.